1. Field of the Invention
The invention relates to apparatus for controlling AC motors and, more particularly, to an energy-saving controller which varies the average voltage supplied to the motor as the load of the motor varies.
2. Background of the Invention
Conservation of electrical energy is becoming increasingly important in the design and application of AC electric motors. To meet the demand for greater efficiency in the operation of such motors, a variety of increasingly sophisticated control apparatus has become available. For example, U.S. Pat. No. 4,052,648 to Nola and NASA Brief No. MFS-23280 describe a control circuit which reduces power loss in an AC induction motor by sensing the power factor, or phase lag between voltage and current, of the motor. This information is used to vary the average voltage supplied to the motor to maintain a constant optimum power factor in the face of changing loads and line voltages.
Modifications and improvements of the circuit described in the aforementioned U.S. Pat. No. 4,052,648 have resulted in a wider variety of useful applications of the basic power factor control circuit as well as significant increases in the protection of motors controlled by such circuits. For example, copending U.S. patent application Ser. No. 161,327, filed June 20, 1980, by the inventor of the present invention describes an energy saving motor controller applicable to multi-phase motors and capable of a variety of additional features such as in-rush current limiting, over-current trip capability, controlled start-up, and phase loss detection. Such controllers provide very satisfactory service in a variety of applications. However, under certain conditions such as extremely high loads and severe transient loads, motor controllers which operate by sensing changes in the power factor can deliver unsatisfactory performance and may even allow the motor to stall. Furthermore, stability problems are sometimes evident with power factor motor controllers under certain specific load conditions. In order to minimize such stability problems, the response time of most power factor motor controllers is therefore somewhat limited. It would be desirable to provide a controller which would exhibit stable performance over a wider operating range and which would also provide faster response to load transients.
In certain industrial environments, higher than normal line voltage conditions may frequently exist. Operating a motor at such higher voltages can result in higher energy consumption than is necessary to properly drive the load. It would be desirable to provide a motor controller which would reduce the energy consumed by an electric motor under conditions of high line voltage.
Some applications such as in the mining and construction industries can result in overhauling, or regenerative, load conditions. This exists, for example, when a crane is lowering a weight and the load attempts to drive the motor at faster than synchronous speed. Power motor controllers may sometimes operate the motor in an undesirable manner under such conditions. It would therefore be desirable to provide a controller which would allow motor operation to actually take advantage of overhauling loads to save energy by feeding power back into the source.
Finally, although power motor controllers provide generally satisfactory service in many applications, it would be desirable to provide equal or improved performance with simpler apparatus.